Logging trucks are conventionally equipped with four scale pads, two of which are on the tractor and two of which are on the trailer. The tractor includes a log bunk pivotally supported therefrom and with a corresponding pair of scale pads disposed beneath opposite sides of the tractor log bunk. The trailer is also equipped with a log bunk and trailer scale pads are disposed under opposite sides of the trailer log bunk. The scale pads, when pressurized, elevate the log bunks and the load supported therefrom. The scale pads are pneumatically operated and are constructed to vent to the ambient atmosphere after being elevated to the uppermost positions. As long as the tractor and trailer are disposed on reasonably level ground, the pressure required to maintain the scale pads elevated below their maximum elevation may be transposed into weight readings whereby the load supported from the bunks may be determined. However, in most instances, the areas in which the trucks are loaded with logs are not level resulting in uneven loading on the scale pads and if the scale pads are tilted and equipped with conventional side mounted pop-off valves, one pad will render a light reading, but in reality the true weight will be greater. The reason for this inaccurate weight reading is because the conventional side mounted pop-off valve is allowed to vent to the ambient atmosphere short of its lift. If a conventional scale pad is tilted downward toward the side of the conventional pop-off valve, the weight reading will be light. The reason for this is that the scale pad is forced to travel further in order to vent to the ambient atmosphere. As a result, the scale pad is over-pressurized and thereby renders a heavy weight reading. Accordingly, a need exists for scale pads which will be capable of rendering a true reading of the loading on the corresponding log bunks, even if the truck being loaded is disposed on uneven ground. Various forms of previously known scale pads and similar structures are disclosed in U.S. Pat. Nos. 2,501,730, 3,327,797, 3,695,100 and 4,162,710.
The instant invention comprises an improvement on the above last-mentioned patent and the improvement resides in modifications which have been made to the pop-off valve assembly. These modifications include structure by which the pop-off valve assembly is more positively fluid-tight sealed relative to the central portion of the flexible diaphragm and structure of the improved valve assembly whereby venting of the lower portion of the valve assembly to the ambient atmosphere is effected at the uppermost limit of movement of the piston portion of the valve assembly.